This invention relates to systems that ascertain what is occupying a vehicle seat for the purpose of deciding if and how air bags should be deployed.
Air bags of occupant protection systems are expensive and in certain circumstances are dangerous. It is therefore desirable to prevent deployment when the seat is empty to save the cost of replacement. It is important to prevent deployment when circumstances do not warrant deployment or when deployment might do more harm than good. When the seat is occupied by a child or by a very small adult it is particularly important to deploy the airbag judiciously. A system is desired that can reliably distinguish a 100 pound adult from a 60 pound child in a child seat even when the belts retaining the child seat are under substantial tension.
Occupant protection systems typically include a xe2x80x9csensor and diagnostic modulexe2x80x9d or xe2x80x9cSDMxe2x80x9d which performs various functions related to sensing the occurrence of a vehicle crash, monitoring various elements of the occupant protection system for proper operation and initiating deployment of occupant protection devices. SDM""s typically include a microprocessor, an accelerometer, an arming sensor, circuitry interconnecting the aforementioned components and switches for initiating deployment of the occupant protection devices. SDMs are often connected to receive input from such as side mounted and forward mounted crash sensors.
Many systems are known that sense properties of the occupant of a seat. Certain of these systems contain elements for weighing the occupant. If the weight is very small it may be assumed that the seat is unoccupied or occupied by a small child, and in either case deployment would not be desired. If the weight is intermediate, say between 25 and 40 kilograms, then the occupant is likely to be a child and whether or not an airbag should be deployed depends on factors such as how energetically the airbag deploys. If the weight is greater than 45 kilograms the seat occupant is likely to be an adult who would be protected by an airbag.
Three types of weight sensing systems for installation in vehicle seats are known: A first type of weight sensing system comprises an array of force sensors located immediately beneath the upholstery material of the seat cushion which operates to measure the pressure of the occupant against the seat at the points where sensors are located. These sensors are typically responsive to small forces applied over a small area and an array of force sensors tells a microprocessor the magnitude and distribution of the force the occupant applies to the cushion. The microprocessor ascertains the weight and other characteristics of the seat occupant from the information provided by the array of force sensors.
A second type of weight sensing system is useful in the type of seat having a fabric covered seat cushion of a foam of a rubbery material supported by a platform. The second type of weight sensing system includes, typically, four force sensors located at the four comers of the platform between the platform and the frame of the seat. The outputs of the four sensors are added to ascertain the total weight being supported by the platform and, therefore, by the seat cushion.
A third type of weight sensing system comprises sensors for sensing stress in structural members of the seat. For example, a chair with a force sensor sensing the weight carried by each leg. The outputs of the four (in the case of the example) sensors are added to ascertain the total weight of the occupant.
The known embodiments of the aforementioned weight sensing systems do not always measure the occupant""s weight accurately and no design is widely accepted. Where the seat belts are attached to the structure of the vehicle, all of the aforementioned weight sensing systems may provide weight readings for a tightly belted child seat that are indistinguishable from weight readings from an adult.
Additionally, all known embodiments of the aforementioned weight sensing systems attempt to measure the total occupant weight but do so with a large margin of error because the feet of most normally seated adults (but not of children and very small adults) rest on the floor so that some of the weight is not sensed.
Force sensors placed immediately beneath the seat upholstery complicate manufacture and may affect the feel of the seat as sensed by the occupant.
Load cells comprising a piston sealingly movable in a cylinder to generate hydraulic pressure are well known. In certain applications there is a need for a load cell that responds to both tension and compression. A sensor based on a piston sealingly movable in a tube must be preloaded with such as spring under tension or compression to maintain a pressure in the liquid that diminishes when tension is applied to the load cell. Known means for providing spring force disadvantageously respond to changes in temperature because liquids typically have larger thermal expansion coefficients than metals which causes the deflection of the spring and therefore the spring force to vary with temperature.
Another disadvantage of load cells comprising a piston sealingly movable in a cylinder is friction between the piston and the cylinder which results from side forces that can result from many causes. Commonly encountered causes are forces caused by differential thermal expansion between the car floor and the seat, relative movement as the seat is being attached to the vehicle, damage to the seat or the car floor and forces resulting from acceleration of the vehicle or actions of the seat occupant. It is particularly important to isolate the piston from angular misalignment between seat parts and car floor parts that occur because of production variations in the parts. A load cell is needed that is inherently insensitive to side forces and angular misalignments.
Seat occupant weight sensing systems responsive to stress in the seat structure must respond only to forces related to the weight of the seat occupant and not to stresses resulting from thermal expansion or attachment to the vehicle. This is not always easily achieved. However, force sensors mounted on the seat structure solve the aforementioned problem of belt forces causing a child to appear to be an adult: By anchoring the seat belts to the seat and placing the force sensors below the belt anchors the belt forces are not included in the weight.
Load cells responsive to applied force by pressurizing a liquid to which a pressure sensor responds may include an absolute pressure sensor. Absolute pressure sensors advantageously are less expensive to manufacture and allow the load cell to be closed to prevent penetration of fluids during events such as flooding or fluid spillage. The output of a load cell comprising an absolute pressure sensor responds to changes in atmospheric pressure. A change in output as a car is driven from sea level to Denver, Colo. will occur and might be on the order of three to ten pounds change in the force sensed by each load cell.
The type of sensor wherein the weight of an occupant sitting on a cushion is transmitted though the cushion and sensed at the platform supporting the cushion may fail to register an accurate weight because a fraction of the occupant""s weight may be supported by the back of the seat rather than on the seat cushion and, therefore, not be sensed. Also, the fraction of the occupant""s weight supported by the seat back varies with the angle to which the back is reclined.
Seat backs that can recline typically expose a larger area of cushion at greater seat back recline angles. This causes the weight of the head and body of an occupant to be applied farther toward the rear of the seat cushion when the seat. back is reclined.
Semiconductor pressure sensors are manufactured in large quantities by micromachining silicon wafers. Designs are based on various technologies and physical principles. Many of these sensors require additional circuitry to achieve a useful function. Typically, an integrated circuit complements the micromachined pressure sensing element. Certain of these sensors are suitable for operation submersed in liquid and operate by sensing the pressure in the liquid. Absolute pressure sensors are typically less expensive than gauge sensors because there is no need to provide a channel connecting the inside of the diaphragm to the atmosphere. Absolute pressure sensors also simplify the design of a load cell based on a semiconductor pressure sensor because it is not necessary to provide a duct from the semiconductor pressure sensor to the outside atmosphere.
Accelerometers responsive to accelerations between plus and minus one or two times the acceleration of gravity are readily available from several suppliers. One supplier is Entran Devices, Inc. of Fairfield, N.J.
Seats in commercial production often include sensors indicating the position of the seat on its track and the amount that the back of the seat is reclined.
Injection stretch blow molding is a highly developed technology for making beverage containers. By this process a hollow piece of injection molded plastic is heated and placed in a form after which an air nozzle extends into the form to stretch it and pressurized air forces the stretched plastic to conform to the mold. This method is sometimes used to make bottles with sides shaped like bellows to stretch like the bellows of an accordion to vary the capacity of the bottle.
It is well known to connect a sensor using only two electrical conductors. In typical systems the sensor simultaneously draws power needed to operate and also draws pulses of current over and above the current required to operate. The width, magnitude or pattern of the pulses indicates the physical measurement.
By the rules of Physics torque is a vector quantity defined with reference to a force and an axis. A torque about an axis is caused by a force (a vector quantity) applied along a line that does not intersect the axis. The torque (also a vector quantity) is the vector product of the force vector and a distance vector from the axis to the line along which the force vector is applied. In the apparatus of the invention only the scaler magnitude of the torque vector is measured and, hereinafter, it is called xe2x80x9ctorquexe2x80x9d. Hereinafter, xe2x80x9ctorquexe2x80x9d about an axis resulting from an applied force vector is defined to be the product of the length of the line from the axis to the force vector that is perpendicular to both the axis and the force vector multiplied by the component of the force vector perpendicular to both the axis and the line.
Belleville springs are washers dished into a slightly conical shape. Certain Belleville springs are called xe2x80x9cconstant force springsxe2x80x9d because the force required to compress the spring is largely independent of the compression over a range of compression distances. The constant force nature of these Belleville springs is often used to maintain a constant spring force in a mechanism where relative movement of parts of the mechanism would cause the force provided by most other spring designs to vary. Belleville springs having a dish height to thickness ratio of about 1.4 are constant force springs when they are compressed to flatness. Belleville springs having dish height to thickness ratios greater than 1.4 have a constant force region of compression at compressions to less than to flatness.
Child seats are made in three types. Infant seats are mounted in a rear facing orientation and are typically intended for infants weighing less than 18 pounds. Child seats are mounted in a forward facing direction and provide cushioned seat cushion and seat back surfaces that are located about three inches away from the vehicle seat cushion and seat back surfaces respectively. They are typically intended for children weighing 18 and 40 pounds but some designs may be mounted in a rear facing orientation for use as an infant seat. The first two types are anchored by the vehicle seat belts which pass through openings in the child seat which keep the vehicle seat belts away from the child and enable the belts to be highly stressed to firmly attach the child seat to the vehicle seat. The first two types of child seat provide their own belts for restraining their occupant. The third type uses the vehicle seat belts to restrain the child and some are intended for children weighing as much as 60 pounds. In the case of the third type the vehicle belts directly or indirectly apply force to the child which makes operation with a large belt tension unlikely because the large force would cause the child discomfort.
Copending application Ser. No. 09/081,194 describes a load cell for generating an electric signal indicating the force applied to the load cell. It has a pressure sensor and a means for converting applied force to pressure whereby it becomes a force sensor. It is preloaded with a constant force spring whereby relative thermal expansion between the liquid and the structural parts of the load cell does not cause the pressure in the liquid to vary. The constant force spring also provides a low friction bearing in the axial direction and resists radial movement between two parts of the load cell.
In vehicle manufacturing there are variations in the orientations of both the parts of the vehicle to which seats are attached and in the parts of the seats that attach to the vehicle. Copending application Ser. No. 09/081,194 describes a load cell for sensing seat occupant weight wherein a constant force spring accommodates angular misalignment between seat elements and vehicle elements that normally occur as a result of manufacturing processes.
Copending application Ser. No. 09/112,727 describes a seat occupant weight sensing system based on torque sensed at the cushion of a seat and two seat occupant weight sensing systems based on torque sensed at the frame of the seat. The seat occupant weight sensing systems it describes are similar in many ways to the system of the present invention except that in FIGS. 11, 12 and 13 ball bearings are used to obtain a low friction bearing and in the present application FIG. 12, 13 and 14 which correspond with FIGS. 11, 12 and 13 in application Ser. No. 09112,727 illustrate using a Belleville spring to obtain the low friction bearing in the manner disclosed in the aforementioned copending application Ser. No. 09/081,194.
Copending application Ser. No. 09/112,727 also discloses a force sensor comprising a liquid filled injection stretch blow molded bottle having bellows shaped sides and a pressure sensor whereby the bottle becomes a force sensor responsive to axial force converted to pressure in the liquid and sensed by the pressure sensor.
A general object of this invention is to provide a seat occupant sensing system offering low cost and superior performance and also to provide a force sensor that is particularly adapted for sensing force derived from the weight of a seat occupant which also overcomes certain disadvantages of the prior art.
In accordance with a first embodiment of the invention, a seat occupant weight sensing system comprises a platform hinged near the front of the seat and supported by a force sensor near the rearmost part of the platform. The force sensor is, therefore, responsive to torque applied to the platform by a seat occupant.
Further, in accordance with the aforementioned first embodiment of the invention, the seat occupant weight sensing system is particularly responsive to weight applied to the rearward part of the seat and is less responsive to weight applied near the forward edge of the seat thereby being less affected by the weight of the lower legs and feet of a normally seated adult and more responsive to the weight of the torso which is the part of the occupant that is to be cushioned by an airbag. Thereby, any inaccuracies in the measured weight are less relevant to deploying an airbag than inaccuracies in the measured weight from a weight sensing system that is more affected by weight applied by the occupant""s feet to the vehicle floor.
Further, in accordance with the aforementioned first embodiment of invention, the hinge extends horizontally and transversely to the vehicle axis whereby the axis of rotation of the platform is near the front of the seat.
Further, in accordance with the aforementioned first embodiment of the invention, a seat back recline indicator enables calculating the occupant weight from the torque and recline angle thereby correcting for weight supported by the seat back. In certain cases of large recline angle the recline angle may dictate that the occupant is located where it would not be useful to deploy an airbag.
Further, in accordance with the aforementioned first embodiment of the invention, the more rearward position of a normally seated occupant when the seat back is somewhat reclined partially or approximately compensates for the downward force the occupant applies to the seat back when the seat back is reclined.
Further, in accordance with a second embodiment of the invention, torque applied to the frame of the seat is measured. A seat back recline sensor and a seat track position sensor provide information that enables calculating the occupant weight from the observed torque thereby enabling determination of occupant weight with only one force sensor. This embodiment may add little cost over the cost of a seat without a force sensor because many seats incorporate a track position sensor and a seat back recline sensor for other purposes. In certain cases the combination of the seat back recline angle and the seat track position may indicate that the seat occupant is located where airbag deployment might not be useful.
Further, in accordance with the invention, the force sensor comprises means for converting force to hydraulic pressure and a pressure sensor provides an electric signal indicating the hydraulic pressure.
Further, in accordance with the invention, a force sensor comprises a tubular neck sealingly accommodating a cylindrical plastic feedthrough having insert molded electrical conductors and a pressure sensor mounted on an end.
Further, in accordance with the invention, the aforementioned force sensor also comprises a piston having a rounded protrusion and further comprises a cover having a protrusion mating with said protrusion and a retaining lip whereby the number of parts and the cost are minimized.
Further, in accordance with the invention, a second embodiment of the aforementioned force sensor comprises an injection stretch blow molded bottle with bellows shaped walls and a neck sealed to the aforementioned cylindrical plastic feedthrough having insert molded electrical conductors and a pressure sensor mounted on an end.
Further, in accordance with the aforementioned second embodiment of the force sensor of the invention the neck of the blow molded bottle is formed with threads using the technology used to form the threads on the neck of a soda bottle and the aforementioned cylindrical plastic feedthrough is formed in the shape of a cap of a soda bottle with mating threads with which it is sealingly attached to the injection stretch blow molded bottle thereby using the same technology that is used to seal the caps onto soda bottles.
Further, in a variation of the aforementioned second embodiment of the force sensor of the invention, the force sensor is preloaded by a constant force Belleville spring. The Belleville spring also prevents radial movement between a part of the force sensor attached to a seat frame and a part of the force sensor attached to the vehicle while allowing axial relative movement between the two parts. Advantageously, the axial movement is substantially free of friction. Also advantageously, some angular misalignment between the part of the vehicle floor to which the seat attaches and the part of the seat that is attached to the vehicle can be accommodated by twisting of the Belleville spring.
Further, in the aforementioned variation of the aforementioned second embodiment of the force sensor of the invention, a clamp prevents separation of the part of the force sensor attached to a seat frame and the part of the force sensor attached to the vehicle thereby preventing the seat from moving upward during violent movement such as occurs on a rough road or during a crash.
Further, in accordance with the invention, two or more force sensors are connected with a microprocessor which adds the outputs of the force sensors to calculate the total force.
Further, in accordance with the invention, an atmospheric pressure sensor informs the microprocessor of the atmospheric pressure which enables the pressure sensors of the force sensors to be absolute pressure sensors and enables the microprocessor to correct for variations in the outputs of the force sensors caused by variations in atmospheric pressure.
Further, in accordance with the invention, changes of the readings of the force sensor are compared with changes of vertical accelerations measured by an accelerometer for ascertaining if the seat contains a child seat.
Further, in accordance with the invention, a switch responsive to a predetermined seat belt tension is provided that closes when the seat belt tension is so great it would be uncomfortable to a human and therefore indicates that the seat is being occupied by a tightly belted child seat.
Further, in accordance with the invention, a sensor responsive to seat belt tension is provided that enables calculating the weight of the seat occupant in the case when seat belt tension is significant.
A complete understanding of this invention may be obtained from the description that follows taken with the accompanying drawings.